1. Field of the Invention
This invention generally relates to a laser device, a laser module, a semiconductor laser and a fabrication method of the semiconductor laser.
2. Description of the Related Art
Generally, a wavelength-changeable semiconductor laser has a semiconductor element having a gain for a laser and a wavelength-selectable semiconductor element. The semiconductor laser has, for example, a Sampled Grating Distributed Feedback Laser (SG-DFB) region and a Sampled Grating Distributed Reflector (SG-DR) region.
The semiconductor laser emits a desirable laser light with use of a vernier effect. That is, the laser light is emitted at a wavelength where a longitudinal mode spectrum of the SG-DFB region corresponds to a reflection spectrum of the SG-DR region, in the semiconductor laser. Therefore, it is possible to emit a desirable laser by controlling the longitudinal mode spectrum of the SG-DFB region and the reflection spectrum of the SG-DR region.
In the semiconductor laser, a phase difference of 90 degrees is, however, generated between a light incoming to the SG-DR region and the SG-DFB region and a light reflected by a diffractive grating of the SG-DR region and the SG-DFB region in a design wavelength range of the diffractive grating. Therefore, a phase difference of 180 degrees is generated between lights transmitting in a resonator in directions opposite to each other, in the design wavelength range. Accordingly, a desirable wavelength light is canceled and it is possible that a laser is not emitted.
On the other hand, the phase difference in the diffractive grating is offset from 90 degrees at a wavelength offset to shorter wavelength or longer wavelength from the design wavelength of the diffractive grating. And it is possible that a single wavelength laser is not emitted.